Digital wireless communication systems have utilized many different types of modulation for transmitting data. One type of modulation which has become popular is frequency shift keyed (FSK) modulation because of its tolerance to fading and multipath interference. The well-known Bluetooth wireless communication system, for example, utilizes two-level FSK (2-FSK) modulation. The well-known HomeRF system utilizes both 2-FSK and four-level FSK (4-FSK) modulation.
Normally in the transmitter before FSK modulation takes place, a low-pass filter is applied to the symbol sequence to be transmitted to limit the transmission bandwidth. This low-pass filter can be either a Gaussian filter such as that specified for Bluetooth and HomeRF high-speed transmission, or can be another type of cost efficient low-pass filter such as in HomeRF low-speed transmission where no exact filter type is specified. For narrow band Frequency Hopping systems such as HomeRF, the transmitter pre-modulation filter causes a significant amount of inter-symbol interference (ISI), making the symbols harder to detect in a receiver. In order not to exacerbate the ISI, prior-art FSK receivers have generally utilized relatively wide-band selectivity filters, thereby reducing adjacent channel rejection. In addition, the low-pass pre-modulation filter whose exact type also is not specified in HomeRF adds still more variation in the received signal, making message recovery in the receiver even more difficult. Additional elements, such as a post-detection filter (PDF) and a maximum likelihood sequence estimation (MLSE) element having four or sixteen states for a 2-FSK channel, and sixteen or sixty-four states for a 4-FSK channel, have been used to compensate for the ISI. Such additional elements add to the cost of the receiver and increase the total power consumption. Also, in the prior art, the templates utilized in the MLSE generally have been designed to match the bandwidth of the transmitter pre-modulation filter. Due to the fact that the transmitter pre-modulation filter in low-speed HomeRF is not specified precisely, the MLSE implementation becomes even more difficult.
Thus, what is needed is an apparatus for receiving and recovering symbols transmitted in an FSK digital wireless channel. The apparatus preferably will achieve high sensitivity and adjacent channel rejection and be sufficiently robust to tolerate the transmitter pre-modulation filter variations without the need for either the PDF or the expensive MLSE element with four or sixteen states for 2-FSK, and, with sixteen or sixty-four-states for 4-FSK.